I think we are talking about differing times in evolving languages. All the letters of Latin are most certainly not pronounced in Spanish. That is why a bunch of words end in -o instead of -us or why no verb conjugation ends in -m or -t anymore.
At some point people in Spain stopped writing Latin and started writing Spanish, at which point spelling matched pronunciation. But that was not today's Spanish. There were big pronunciation and spelling changes around the 15th century IIRC - see this article for some hints of how it used to look and sound. https://en.wikipedia.org/wiki/Old_Spanish_language
The current "spelling matches pronunciation" thing only occurred because the spelling was updated when pronunciation changed. It is kind of a time capsule, needs to be periodically "maintained", not innate to the language.
Also important to note that there are places where not every letter is pronounced in modern Spanish. Eg. Many accents aspirate or omit S in some positions. Some people omit D between vowels.
1) z and s, or ce and se, are pronounced the same everywhere except some parts of Spain. I guess not an example of silent letters but still, an example of a distinction that existed historically/etymologically but that no longer does in the spoken language for the vast majority of speakers.
2) The h is silent in every dialect despite having usually represented a real sound in Latin
> z and s, or ce and se, are pronounced the same everywhere except some parts of Spain.
I didn't cite that because I'm already blabbing too much about this topic, and I didn't want to overwhelm people, but yes. Also, as cited by the sibling comment, the merger of /b/ and /v/. Also, in many accents, but not all, Y and LL are the same. Also, consonants like /b/ or /g/ are sometimes swallowed when followed by /w/.
Also <b> and <v> have merged in a lot of versions of Spanish. The <v> is postclassical but represents /w/. A minimal pair for these in Latin is bis 'twice' and vis 'force'. (Well, not quite, because of different vowel lengths... let's try bilis 'bile' and vilis 'vile'.)
So, I can't speak to Spanish, but I can speak to French. There, the suffixes that are silent in Metropolitan French are still pronounced in some regional dialects, but the grammar doesn't really change (that I've noticed).
That leads me to think that it could just as easily be that grammar changes render the agglutination superfluous, and it subsequently withers away like a vestigial organ.
At some point people in Spain stopped writing Latin and started writing Spanish, at which point spelling matched pronunciation. But that was not today's Spanish. There were big pronunciation and spelling changes around the 15th century IIRC - see this article for some hints of how it used to look and sound. https://en.wikipedia.org/wiki/Old_Spanish_language
The current "spelling matches pronunciation" thing only occurred because the spelling was updated when pronunciation changed. It is kind of a time capsule, needs to be periodically "maintained", not innate to the language.
Also important to note that there are places where not every letter is pronounced in modern Spanish. Eg. Many accents aspirate or omit S in some positions. Some people omit D between vowels.